arXiv subjects

J. E. Moore

Publications and source records attributed to J. E. Moore.

46 records · Page 3Linked to original sources

arXiv2005

Observation of spin Coulomb drag in a two-dimensional electron gas

An electron propagating through a solid carries spin angular momentum in addition to its mass and charge. Of late there has been considerable interest in developing electronic devices based on the transport of spin, which offer potential advantages in dissipation, size, and speed over charge-based devices. However, these advantages bring with them additional complexity. Because each electron carries a single, fixed value (-e) of charge, the electrical current carried by a gas of electrons is simply proportional to its total momentum. A fundamental consequence is that the charge current is not affected by interactions that conserve total momentum, notably collisions among the electrons themselves. In contrast, the electron's spin along a given spatial direction can take on two values, "up" and "down", so that the spin current and momentum need not be proportional. Although the transport of spin polarization is not protected by momentum conservation, it has been widely assumed that, like the charge current, spin current is unaffected by electron-electron (e-e) interactions. Here we demonstrate experimentally not only that this assumption is invalid, but that over a broad range of temperature and electron density, the flow of spin polarization in a two-dimensional gas of electrons is controlled by the rate of e-e collisions.

cond-mat.mes-hall↗

arXiv2005

Stability of the quantum spin Hall effect: effects of interactions, disorder, and Z_2 topology

The stability to interactions and disorder of the quantum spin Hall effect (QSHE) proposed for time-reversal-invariant 2D systems is discussed. The QSHE requires an energy gap in the bulk and gapless edge modes that conduct spin-up and spin-down excitations in opposite directions. When the number of Kramers pairs of edge modes is odd, certain one-particle scattering processes are forbidden due to a topological $\mathbb{Z}_2$ index. We show that in a many-body description, there are other scattering processes that can localize the edge modes and destroy the QSHE: the region of stability for both classes of models (even or odd number of Kramers pairs) is obtained explicitly in the chiral boson theory. For a single Kramers pair the QSHE is stable to weak interactions and disorder, while for two Kramers pairs it is not; however, the two-pair case can be stabilized by {\it either} finite attractive or repulsive interactions. For the simplest case of a single pair of edge modes, it is shown that changing the screening length in an edge with screened Coulomb interactions can be used to drive a phase transition between the QSHE state and the ordinary insulator.

cond-mat.mes-hall↗

arXiv2005

Geometric criticality between plaquette phases in integer-spin kagome XXZ antiferromagnets

The phase diagram of the uniaxially anisotropic $s=1$ antiferromagnet on the kagomé lattice includes a critical line exactly described by the classical three-color model. This line is distinct from the standard geometric classical criticality that appears in the classical limit ($s \to \infty$) of the 2D XY model; the $s=1$ geometric T=0 critical line separates two unconventional plaquette-ordered phases that survive to nonzero temperature. The experimentally important correlations at finite temperature and the nature of the transitions into these ordered phases are obtained using the mapping to the three-color model and a combination of perturbation theory and a variational ansatz for the ordered phases. The ordered phases show sixfold symmetry breaking and are similar to phases proposed for the honeycomb lattice dimer model and $s=1/2$ $XXZ$ model. The same mapping and phase transition can be realized also for integer spins $s \geq 2$ but then require strong on-site anisotropy in the Hamiltonian.

cond-mat.str-el↗

arXiv2005

Ordering near the percolation threshold in models of 2D interacting bosons with quenched dilution

Randomly diluted quantum boson and spin models in two dimensions combine the physics of classical percolation with the well-known dimensionality dependence of ordering in quantum lattice models. This combination is rather subtle for models that order in two dimensions but have no true order in one dimension, as the percolation cluster near threshold is a fractal of dimension between 1 and 2: two experimentally relevant examples are the O(2) quantum rotor and the Heisenberg antiferromagnet. We study two analytic descriptions of the O(2) quantum rotor near the percolation threshold. First a spin-wave expansion is shown to predict long-ranged order, but there are statistically rare points on the cluster that violate the standard assumptions of spin-wave theory. A real-space renormalization group (RSRG) approach is then used to understand how these rare points modify ordering of the O(2) rotor. A new class of fixed points of the RSRG equations for disordered 1D bosons is identified and shown to support the existence of long-range order on the percolation backbone in two dimensions. These results are relevant to experiments on bosons in optical lattices and superconducting arrays, and also (qualitatively) for the diluted Heisenberg antiferromagnet.

cond-mat.dis-nn↗

arXiv2005

Nonequilibrium charge density wave ordering from anomalous velocity in itinerant helical magnets

The Karplus-Luttinger anomalous velocity is shown to lead to electric-field-induced charge accumulation in nearly ferromagnetic noncollinear magnets with itinerant electrons, like $MnSi$. For helical magnetic ordering, the balance between this accumulation and the Coulomb interaction leads to a nonequilibrium charge density wave state with the period of the helix, even when such accumulation is forbidden by an approximate gauge-like symmetry in the absence of electric field. We compute the strength of such charge accumulation as an example of how unexpected many-electron physics is induced by the inclusion of the one-electron Karplus-Luttinger term whenever the local exchange field felt by conduction electrons does not satisfy the current-free Maxwell equations.

cond-mat.str-el↗

arXiv2004

Dimensional reduction in superconducting arrays and frustrated magnets

Some frustrated magnets and superconducting arrays possess unusual symmetries that cause the free energy or other physics of a $D$-dimensional quantum or classical problem to be that of a different problem in a reduced dimension $d<D$. Examples in two spatial dimensions include the square-lattice $p+ip$ superconducting array, the Heisenberg antiferromagnet on the checkerboard lattice (studied by a combination of 1/S expansion and numerical transfer matrix), and the ring-exchange superconducting array. Physical consequences are discussed both for ``weak'' dimensional reduction, which appears only in the ground state degeneracy, and ``strong'' dimensional reduction, which applies throughout the phase diagram. The ``strong'' dimensional reduction cases have the full lattice symmetry and do not decouple into independent chains, but their phase diagrams, self-dualities, and correlation functions indicate a reduced effective dimensionality. We find a general phase diagram for quantum dimensional reduction models in two quantum dimensions with $N$-fold anisotropy, and obtain the Kosterlitz-Thouless-like phase transition as a deconfinement of dipoles of 3D solitons.

cond-mat.str-el↗

arXiv2004

Entanglement entropy of random quantum critical points in one dimension

For quantum critical spin chains without disorder, it is known that the entanglement of a segment of N>>1 spins with the remainder is logarithmic in N with a prefactor fixed by the central charge of the associated conformal field theory. We show that for a class of strongly random quantum spin chains, the same logarithmic scaling holds for mean entanglement at criticality and defines a critical entropy equivalent to central charge in the pure case. This effective central charge is obtained for Heisenberg, XX, and quantum Ising chains using an analytic real-space renormalization group approach believed to be asymptotically exact. For these random chains, the effective universal central charge is characteristic of a universality class and is consistent with a c-theorem.

cond-mat.dis-nn↗

arXiv2003

Quantum destruction of stiffness in diluted antiferromagnets and superconductors

The reduction of 2D superconducting or antiferromagnetic order by random dilution is studied as a model for the 2D diluted Heisenberg antiferromagnet (DHAF) La$_2$Cu$_{1-p}$(Zn,Mg)$_p$O$_4$ and randomly inhomogeneous 2D suerconductors. We show in simplified models that long-range order can persist at the percolation threshold despite the presence of disordered one-dimensional segments, contrary to the classical case. When long-range order persists to the percolation threshold, charging effects (in the superconductor) or frustrating interactions (in the antiferromagnet) can dramatically modify the stiffness of the order. This quantum destruction of stiffness is used to model neutron scattering data on La$_2$Cu$_{1-p}$(Zn,Mg)$_p$O$_4$. In a certain simplified model, there is a sharp stiffness transition between ``stiff'' and ``floppy'' ordered phases.

cond-mat↗

arXiv2003

Electronic-vibrational coupling in single-molecule devices

Experiments studying vibrational effects on electronic transport through single molecules have observed several seemingly inconsistent behaviors, ranging from up to 30 harmonics of a vibrational frequency in one experiment, to an absence of higher-harmonic peaks in another. We study the different manifestations of electronic-vibrational coupling in inelastic and elastic electron transport through single molecules. For the case of inelastic transport, higher harmonics are shown to be damped by additional small factors beyond powers of the electron-vibration coupling constant $λ$. Two mechanisms greatly increase the size of secondary peaks in inelastic transport: coupling between electron transport and spatial motion of the molecule, and the ``pumping'' of higher vibrational modes of the molecule when vibrational excitations do not completely relax between electron transits.

cond-mat.mes-hall↗

arXiv1996

Edge Excitations of the $ν= 2/3$ Spin-Singlet Quantum Hall State

The spectrum of edge excitations is derived for the spin-unpolarized $ν= 2$ and $ν= 2/3$ FQHE. Numerical diagonalization of a system of six electrons on a disc confirms that the edge $ν= 2/3$ spin-singlet FQHE state consists of oppositely directed spin and charge branches on the same physical edge. The highly correlated $ν= 2/3$ singlet edge is shown to have the same spin branch as the $ν= 2$ singlet edge, providing evidence that the same $SU(2)_{k = 1}$ Kac-Moody algebra describes all unmixed spin branches. The spin and charge branches of the singlet state at $ν= 2/3$ are less coupled than the two branches of the spin-polarized state at the same filling factor, suggesting that the conductance along an edge may increase sharply across the polarized-unpolarized transition.

cond-mat↗